In a machine tool, when controlling the angular position of the spindle or spindle motor, that is, when drilling or contouring during C-axis control, it is necessary that the spindle be rotated smoothly at a superlow speed and have a high torque. Further, for normal cutting machining etc. high speed rotation and a high cutting power are required.
Therefore, in the past, two motors were provided for driving the spindle. One was the spindle motor for rotating the spindle at a high speed for cutting machining etc. The other was a C-axis control motor having a detector. This C-axis control motor had an output shaft affixed to a worm. A worm gear engaged with the worm was affixed to the spindle. In this way, during C-axis control, the C-axis control motor was driven to rotate the spindle at a superlow speed. To switch to normal cutting machining etc. after this, the above-mentioned C-axis control motor had to be retracted to a retraction position and the spindle motor driven.
However, as mentioned above, two types of motors had to be provided and, further, worm gears, retraction mechanisms, etc. were necessary. Therefore, the apparatus can not be said to have been the best in space or cost.